Unit 1 Section 2: Cell membranes - the basics

Question 1

Purpose of cell membrane

Answer 1

controls which substances enter and leave the cell (partially permeable) also allowing recognition by other cells

Question 2

purpose of membranes around organelles

Answer 2

dividing the main cell into different compartments, e.g. substances needed for respiration are kept inside the mitochondria by it’s outer membrane

Question 3

some organelle membranes are folded to…

Answer 3

increase surface area and making chemical reactions more efficient, e.g. the inner membrane of mitochondria contains enzymes for respiration, it has a large surface area increasing the number of enzymes present making respiration more efficient

Question 4

vesicles

Answer 4

membranes can form vesicles to transport substances around the cell, e.g. proteins are transported between the RER and Golgi apparatus during protein synthesis, in vesicles

Question 5

membranes within the cell are also..

Answer 5

Partially permeable to select what enters and leaves the organelle, e.g. RNA can leave the nucleus via the nuclear membrane(envelope) however DNA is too large

Question 6

all membranes mainly compose of…

Answer 6

lipids(phospholipids), proteins and carbohydrates.

Question 7

cell surface membrane model is called

Answer 7

fluid mosaic model

Question 8

the fluid mosaic model

Answer 8

phospholipid molecules form a continuous double layer (bilayer). the phospholipids are moving and so the bilayer is fluid. Protein molecules are scattered through the bilayer sometimes with a polysaccharide chain (carbohydrate) attached- these are called glycoproteins and glycolipids when attached to a lipid. cholesterol is also present within the bilayer

Question 9

phospholipids

Answer 9

form a barrier to water soluble (dissolved) substances.

• have a phosphate, hydrophilic (attracts water) head.
• have a lipid, hydrophobic (repels water) tail
• centre of bilayer is hydrophobic so water soluble substances- such as ions & polar molecules- cannot difuse through it.
• small non-polar substances and water can diffuse through the membrane

Question 10

cholesterol

Answer 10

• gives the membrane stability
• type of fat that’s present in all cell membranes (except bacteria)
• fits between the phohpholipids
• binds to hydrophobic tails forcing them to pach closely together. this makes the membrane less fluid and more stable.
• also has hydrophobic region so provides further barrier against polar substances

Question 11

Proteins

Answer 11

• Control what enters and leaves the cell
• some form channels
• others are called carrier
• react as receptors in cell signalling, when a molecule binds to the protein a chemical reaction is triggered inside the cell

Question 12

glycol lipids/proteins

Answer 12

• stabalise the membrane by forming hydrogen bonds with surrounding water
• act as receptors for messenger molecules
• antigens involved in self-recognition

Question 13

temperature

Answer 13

affects how much the phospholipids can move affecting the membrane structure.

Question 14

below 0’C

Answer 14

phospholipids don’t have much energy so they can’t move very much, paked closely together so membrane becomes rigid. carrier (& channel) proteins denature, increasing permeability. Ice crystals may form and peirce thhe membrane making it permeable when it thaws.

Question 15

0-45’C

Answer 15

phospholipids can move and aren’t packed as tightly. increased permeability as phospholipids move about more

Question 16

above 45’C

Answer 16

bilayer starts to melt and becomes more permeable. water inside the cell expands putting pressure on the membrane. Channel )&carrier) proteins denature, increasing the permeability of the cell.